Electricity transmission and distribution networks (or grids) must balance the generation of electricity with the demand from consumers. This is normally achieved by modulating the generation side (supply side) by turning power stations on and off, and running some at reduced load. As most existing thermal and nuclear power stations are most efficient when run continuously at full load, there is an efficiency penalty in balancing the supply side in this way. The expected introduction of significant intermittent renewable generation capacity, such as wind turbines and solar collectors, to the networks will further complicate the balancing of the grids, by creating uncertainty in the availability of parts of the generation fleet. A means of storing energy during periods of low demand for later use during periods of high demand, or during low output from intermittent generators, would be of major benefit in balancing the grid and providing security of supply.
Power storage devices have three phases of operation: charge, store and discharge. Power storage devices generate power (discharge) on a highly intermittent basis when there is a shortage of generating capacity on the transmission and distribution network. This can be signalled to the storage device operator by a high price for electricity in the local power market or by a request from the organisation responsible for the operating of the network for additional capacity. In some countries, such as the United Kingdom, the network operator enters into contracts for the supply of back-up reserves to the network with operators of power plants with a rapid start capability. Such contracts can cover months or even years but typically the time the power provider will be operating (generating power) is very short. This is illustrated in FIG. 1 which shows a typical operating profile for a storage device. In addition, a storage device can provide an additional service in providing additional load at times of oversupply of power to the grid from intermittent renewable generators. Wind speeds are often high overnight when demand is low. The network operator must either arrange for additional demand on the network to utilise the excess supply, through low energy price signals or specific contracts with consumers or constrain the supply of power from other stations or the wind farms. In some cases, especially in markets where wind generators are subsidised, the network operator will have to pay the wind farm operators to ‘turn off’ the wind farm. A storage device offers the network operator a useful additional load that can be used to balance the grid in times of excess supply.
A number of storage technologies have been developed, including pumped hydro, compressed air energy storage (CAES) and batteries. Pumped hydro is the most established energy storage technology. It stores hydraulic potential energy by pumping water from a lower reservoir to an elevated reservoir (the charge phase), using low cost electricity during periods of low demand. The water is held in the reservoir until a period of high demand (the store phase). At the time of peak demand, and peak price, the water is released through a turbine which generates electricity (the discharge phase). Pumped hydro provides a high efficiency, relatively low operating cost means of storing electricity. The requirement for two large reservoirs at different elevations, and associated high civil engineering content, can cause installed costs to be very high and limits the number of suitable sites available, many of which have already been exploited.
CAES uses the potential energy of compressed air to store electricity. Low cost electricity is used to compress air which is then stored in a large storage vessel (usually an underground cavern). During the discharge phase, the air is released from the storage vessel, heated and then expanded through a turbine which drives a generator to produce electricity. CAES theoretically provides a relatively high efficiency and low capital cost solution for electrical energy storage. However CAES is constrained by the requirement for a large storage vessel for the stored air.
Batteries store electricity as chemical potential energy, and can respond rapidly to load changes enhancing system stability. They are not geographically constrained in the way that pumped hydro and CAES are, however they are relatively high cost, and their efficiency declines with time, limiting their useful lifetime.
For a storage device to be commercially viable the following factors are important: capital cost per MW (power capacity), MWH (energy capacity), round trip cycle efficiency and lifetime with respect to the number of charge and discharge cycles that can be expected from the initial investment. For widespread utility scale applications it is also important that the storage device is geographically mobile.
Each of the technologies described above has advantages and disadvantages against the above criteria. A further storage technology which offers a number of advantages is the storage of energy using a cryogen such as liquid air, or cryogenic power storage. In the charge phase, low cost electricity at periods of low demand or excess supply from intermittent renewable generators is used to liquefy air, which is then stored as a cryogenic fluid in a storage tank, and subsequently released to drive a turbine and produce electricity during the discharge or power recovery phase. The technology relies on the energy potential of the temperature differential between air in its liquid phase and gas phase at ambient. The advantages of storing energy in liquid air are that liquid air is energy dense compared to compressed air and is stored at low pressure; hence the storage is considerably cheaper; such systems are not geographically constrained as storage tanks are relatively small and readily located and overall capital costs are low.
WO2007-096656A1 and GB1100569.1 disclose Cryogenic Power Storage Devices (CPSD) that utilise a cryogenic fluid, such as liquid air or liquid nitrogen, as the storage medium to store energy as thermal potential energy for providing power storage and network support services to electricity transmission and distribution networks. The Cryogenic Energy System (CES) described in WO2007-096656A1 is a first type of Cryogenic Power Storage Device (CPSD) and is a fully integrated storage device that requires only electricity and, optionally, heat as an input. The cryogenset described in GB1100569.1 is a second type of Cryogenic Power Storage Device (CPSD) and is a simplified storage/power generation device that uses cryogenic fluid manufactured by an industrial gas liquefaction plant remote from the cryogenset, which is delivered to the cryogenset site either by pipeline or tanker.
The key difference between the CES and the cryogenset is that the integrated CES cycle allows the capture of cold energy used to evaporate and heat the cryogen during the power recovery (discharge) phase, which is stored and then used during the charging phase to enhance the production of the liquid air—a concept known as cold recovery. The inventors have found that by capturing and recycling this cold energy, the round trip efficiency of the CES may be double that of the cryogenset.
The present invention addresses the practical implementation of cold recycle within the CES such that the amount of cold recovered and recycled, and hence the round trip efficiency of the overall cycle, is maximised in a practical, low cost manner.
A number of publications describe similar CES devices. These include:                1. U.S. Pat. No. 6,920,759 B2        2. E. M. Smith et al.; “Storage of Electrical Energy Using Supercritical Liquid Air” and Discussion thereof; Proc Instn Mech Engrs Vol 191 27/77, p. 289-298, D57-D65; 1977        
These two publications describe similar cycles, shown schematically in FIG. 2. Cryogenic liquid is stored in a storage tank 100. During the power recovery phase of the cycle, the cryogenic liquid is first pumped to high pressure using a pump 105. The cryogenic liquid is then evaporated, and the cold thermal energy is stored in a regenerator, or thermal store 102. Power is then recovered from the resultant gaseous flow through a turbine 106. During the re-liquefaction phase of the cycle the liquid storage tank 100 is replenished; high pressure warm gas is circulated through the cold regenerator and expanded through an expansion valve 101 to manufacture liquid which is stored in the storage tank 100.
The present inventors have noticed that in the CES processes described in the current state of the art, the transfer of thermal energy to the thermal store from a cryogenic liquid during power recovery, or gaseous flow during liquefaction, is done at high pressure. This has the advantage of high potential thermal efficiency, especially if the heat is transferred directly from the working fluid to the thermal store. Two high pressure storage design concepts have been proposed in the past:                1. Direct contact of the working fluid with the storage media (as described by E. M Smith et al (ibid.));        2. Indirect contact of the working fluid with the storage media (as described in U.S. Pat. No. 6,920,759 B2).        
The first concept has the advantage of high potential thermal performance in that the heating or cooling fluids are in direct contact with the storage media, typically steel rods as described by Smith et al., or a packed bed of rock particles. The present inventors believe commercial CES systems will need to be at utility scale, of a minimum of 10 MW and preferably 100 MW or higher. The thermal store will therefore be large, having a linear dimension of 10 m or more. The practical difficulties of manufacturing a low cost pressure containment vessel capable of withstanding pressures of at least 100 bar or more typically 150 bar are in the inventors' opinion prohibitive. For this reason, the inventors believe this approach is not viable.
With the second concept, the pressure containment issue is largely resolved in that only the tubes containing the heat transfer fluid are at high pressure. A typical design is shown schematically in FIG. 3. The heat transfer fluid is contained in high pressure tubes 202 and thermal energy is transferred through the tubes by conduction to the storage media 203 which would typically be water, rock chips or concrete. A low cost containment vessel 204 is possible as the vessel only needs to provide mechanical support for the storage media and will operate at low pressure. In the opinion of the present inventors, this design has a number of issues. Firstly, the design will be complex, requiring many high pressure welded joints to be fabricated, yielding an expensive component. Secondly, the thermal resistance between the high pressure tubes and storage media could result in temperature gradients normal to the flow path and poor thermal performance.
Equipment is classified as pressurised equipment by authorised authorities when it functions at a gauge pressure above 4 bar. Equipment functioning at a pressure less than 4 bar is classified as pressurised but can be type approved, and does not require examination by an authorised authority. With a pressure less than 0.5 bar gauge, equipment is not classified as pressure equipment.
The present invention therefore addresses the problem of how to effectively recover the cold thermal energy from the power recovery process, store the recovered cold thermal energy, and effectively utilise the recovered cold thermal energy to reduce the energy cost of manufacturing more cryogen for subsequent storage and re-use in the power recovery process, enhancing the overall (round trip) efficiency of the energy storage system.
The present invention also addresses the problem of how to effectively recover thermal energy from the liquefaction process, store the recovered thermal energy, and effectively utilise the recovered thermal energy to increase output of the power recover process, again enhancing the overall (round trip) efficiency of the energy storage system.